erts: Allow erlang:load_nif/2 to load a NIF from an in-memory binary

Add support for passing `#{memory => NifSO::binary(),
filename => string():binary()}` as the first argument to erlang:load_nif/2.
When a map is given with the 'memory' key/value present, the NIF shared
object is loaded directly from the binary image in memory instead of
from the file system. If the map only contains the 'filename' key/value
it'll revert to the default behavior of loading the SO from file.

This addition is needed so that escripts can package SO dependencies
inside and be distributed as a single executable.

Implementation
--------------
* erts_dlopen_mem() — new static helper in erl_nif.c (Linux/POSIX only).
  On Linux >= 3.17 it uses memfd_create(2) to create an anonymous
  in-memory file, writes the SO image into it, and calls dlopen(3) via
  /proc/self/fd/<fd>.  On older Linux kernels and other POSIX systems it
  falls back to shm_open(3) + /dev/shm/<name>.  The implementation is
  derived from the https://github.com/saleyn/memfd_create proof of
  concept.

* erts_load_nif() — extended argument parsing:
  - Plain filename() continues to work unchanged.
  - #{memory => Binary, filename => Filename} map: Binary is extracted via
    erts_get_aligned_binary_bytes(), passed to erts_dlopen_mem(), and
    the resulting dlopen handle is used directly.  The Filename string
    serves only as a label inside the memory-backed file descriptor.
  - The open-from-memory step is performed before the existing else-if
    validation chain (version checks, module-name check, create_lib)
    so both code paths share the same validation and bookkeeping.
  - erts_sys_ddll_open() is skipped (via !load_from_mem guard) when a
    handle was already obtained from memory.

* The feature is guarded by #if defined(HAVE_DLOPEN) && defined(__unix__).
  On unsupported platforms load_nif/2 returns {error,{load_failed,…}}.

Testing
-------
* nif_SUITE: new test case load_nif_from_mem
  - Happy path: reads nif_mod.1.so into a binary, calls
    erlang:load_nif(#{filename => Path, memory => Binary}, []) via
    nif_mod:load_nif_lib_from_mem/3, and asserts lib_version() == 1.
  - Error path: non-binary second element returns {error,{bad_lib,_}}.

* nif_mod.erl: new exported helper load_nif_lib_from_mem/3 so that the
  load_nif call site lives inside the NIF module (required by the BIF).

Author: saleyn

.gitignore

@@ -213,6 +213,7 @@ JAVADOC-GENERATED
 /erts/emulator/test/*_native_SUITE.erl
 /erts/emulator/test/*_SUITE_data/Makefile
 /erts/emulator/test/*_stripped_types_SUITE.erl
+/erts/emulator/test/nif_SUITE_data/*.so
 /erts/test/install_SUITE_data/install_bin
 /erts/test/autoimport_SUITE_data/erlang.xml
 /erts/emulator/make_test_dir/

erts/emulator/beam/atom.names

@@ -311,6 +311,7 @@ atom extra
 atom false
 atom fcgi
 atom fd
+atom filename
 atom first
 atom firstline
 atom flags