ash/hush: enable full shell feature set; fix missing builtins on old busybox

Both recipes start from `make allnoconfig` and enable a curated CONFIG list.
This fixes missing builtins, fills out the feature set, and fixes a
reproducibility bug.

Missing builtins on old versions: BusyBox renamed several symbols at 1.27.2
(ASH_BUILTIN_ECHO/PRINTF/TEST -> ASH_ECHO/PRINTF/TEST, SH_MATH_SUPPORT[_64] ->
FEATURE_SH_MATH[_64]). The lists used only the modern names, so the
append+oldconfig flow silently dropped them on <=1.26.2 and the 1.2.x island:
ash there shipped with no echo/printf/test builtin (and, being non-standalone,
no /bin/echo fallback) and no arithmetic; hush had no $(( )) on those versions.
Both legacy and modern names are now listed so each applies where it exists.
(Old hush echo/printf/test are unconditional, so they need no symbol.)

Completed the feature set: every CONFIG-gated builtin is enabled (getopts, help,
command, readonly, times, type, [[, source, let, ...), plus ASH/HUSH_BASH_COMPAT,
brace expansion, $LINENO, interactive line editing (history, tab completion,
reverse search, vi mode) and Unicode. hush previously had no arithmetic at all
(FEATURE_SH_MATH was unset) and no aliases.

Config/build fixes required by the new features, across all eras:
- Pin the NEW int symbols (FEATURE_EDITING_MAX_LEN/HISTORY, SUBST_WCHAR): make
  oldconfig prompts for them and aborts on the build's closed stdin otherwise.
- Stop sed-forcing the SH_IS "sh alias" choice: setting a choice member leaves
  two =y and makes oldconfig re-prompt the choice. Irrelevant for our
  directly-invoked binary anyway.
- Drop FEATURE_USERNAME_COMPLETION: old busybox complete_username() uses
  getpwent_r(), a glibc extension musl lacks, breaking the static link.

Reproducibility fix: ash and hush share build_srcdir (busybox/<version>), so
`rm -rf "${build_srcdir}"` is now done before extraction. Without it, building
both shells of one version in a single container (`shvr.sh build ash_X hush_X`,
as the Dockerfile does) leaves the first shell's build tree in place; on ancient
busybox `make` reuses stale objects and the second binary is non-reproducible
(observed as a hush_1.2.2.1 checksum mismatch vs CI's isolated build).

Verified by building real binaries across 1.37.0, 1.21.1 and 1.2.2 (echo
regression fixed, hush arithmetic fixed, builtins + line editing working), a
full 22-version matrix build on both arches (44 targets each, zero errors), and
regenerating checksums/build/{amd64,arm64} for all 44 ash/hush targets; both
arches verify clean. HUSH_ALIAS exists only in busybox 1.38.0+ (harmlessly
dropped on older).

Author: alganet

checksums/build/amd64/ash_1.19.4/bin/ash.sha256sums

@@ -1 +1 @@
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checksums/build/amd64/ash_1.2.2.1/bin/ash.sha256sums

@@ -1 +1 @@
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+a9e80b12e40b44fbead21ccaf4d5514b2b658143eff0b67b1167514b6029cfb5  ash

checksums/build/amd64/ash_1.2.2/bin/ash.sha256sums

@@ -1 +1 @@
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checksums/build/amd64/ash_1.20.2/bin/ash.sha256sums

@@ -1 +1 @@
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+d87efa8d699c3e39ddf1ebe6e7f30c392d7999347eebff8903c8313e937788ee  ash

checksums/build/amd64/ash_1.21.1/bin/ash.sha256sums

@@ -1 +1 @@
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+0298974afcc5bfc19e37d8c2e4aa9e6e357d2497e504aec1ccf78102f5ccc1cf  ash

checksums/build/amd64/ash_1.22.1/bin/ash.sha256sums

@@ -1 +1 @@
-1c38777fc76d1e24404acdc3045aba54e0450960bf120d178e66959804d7eccd  ash
+3acf2a1f8445dc817a2852bc36f9f637a73137f94dfd935d6c7b58d3a5482405  ash

checksums/build/amd64/ash_1.23.2/bin/ash.sha256sums

@@ -1 +1 @@
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+1221916b9a9bb2b5ee8d3108542a278643ab5d0849ca8856eda8941d3f3e8d72  ash

checksums/build/amd64/ash_1.24.2/bin/ash.sha256sums

@@ -1 +1 @@
-0e24a55ca1b3d519df78db051c52282f1b62a3bbf1f1c3e6b84c2ebb2ead9aca  ash
+af1950a26d94682f7e65111d0bc1493e4188abe92aaaf9d7589d49df6c341745  ash

checksums/build/amd64/ash_1.25.1/bin/ash.sha256sums

@@ -1 +1 @@
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+54a92d1cd67bc50fb84f37ea388844a1d205d52716183be28f5eec0b6093693c  ash

checksums/build/amd64/ash_1.26.2/bin/ash.sha256sums

@@ -1 +1 @@
-85e0caaf8e3e30c2c94eeee9dea6a6cb4069974457a22c36f58953c516f1ec05  ash
+2d1f22685eb56b1ba2e363852e6d9d1641353841e9b34abd5a0233e06b21293f  ash