http: declare unsigned-char alphabet in ragel parsers

ragel 6.10 -G2 generates incorrect code for the obs_text = 0x80..0xff
range when the build host's default `char` is unsigned (notably
aarch64). The state machine collapses byte 127 (DEL) and the
legitimately allowed obs_text range into a single forbidden interval
(`127 <= *p -> goto error`), so any HTTP field value containing
bytes 0x80..0xff is rejected even though RFC 7230 §3.2.6 permits
them.

x86_64 happens to produce correct code via a different range-merging
path. The .rl source is identical; only the host's signed-or-
unsigned-char default flips ragel's codegen.

The fix uses two ragel directives per machine:

  alphtype unsigned char;       # picks the correct codegen path
  getkey ((unsigned char)(*p)); # forces fetched bytes unsigned

`alphtype unsigned char;` is the load-bearing fix: it makes ragel
treat the alphabet as 0..255 regardless of host, picking the
correct emit that lists `case 127: goto st0` plus a 0..31 ctrl-range
check while leaving 0x80..0xff fall through to the accept branch.

`getkey ((unsigned char)(*p))` is required because ragel now emits
character literals with the `u` suffix (e.g. `case 13u:`). On x86 the
input pointer's `*p` is signed char, and comparing a signed-char to an
unsigned literal triggers -Wsign-compare which seastar promotes to an
error. The getkey directive casts the fetched byte to unsigned so the
comparisons are unsigned on both sides.

Affects seastar's HTTP server (http_request_parser via httpd.hh) and
HTTP client (http_response_parser via client.cc) on arm64; the
chunked-transfer trailer parser (http_chunk_trailer_parser) inherits
the same fix.

Unblocks three existing tests that fail on aarch64 once the obs_text
input is exercised:
  - tests/unit/chunk_parsers_test.cc :: test_trailer_headers_parsing
  - tests/unit/request_parser_test.cc :: test_header_parsing
  - tests/unit/httpd_test.cc :: test_full_chunk_format

Fixes scylladb#3413.

Author: travisdowns

src/http/chunk_parsers.rl

@@ -32,6 +32,9 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# alphtype + getkey: workaround for #3413
+alphtype unsigned char;
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);
@@ -152,6 +155,9 @@ public:
 %%{
 
 access _fsm_;
+# alphtype + getkey: workaround for #3413
+alphtype unsigned char;
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);

src/http/request_parser.rl

@@ -33,6 +33,9 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# alphtype + getkey: workaround for #3413
+alphtype unsigned char;
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);

src/http/response_parser.rl

@@ -35,6 +35,9 @@ namespace seastar {
 %%{
 
 access _fsm_;
+# alphtype + getkey: workaround for #3413
+alphtype unsigned char;
+getkey ((unsigned char)(*p));
 
 action mark {
     g.mark_start(p);