Harden numeric parsing and command overrides

[M0nd0R]

Summary

• validate numeric option strings with full strtoumax(..., base 0) consumption instead of accepting any digits/x mixture
• reject malformed UID/GID inputs such as x, 0x, 08, and 1x instead of letting them silently parse as UID/GID 0 or a truncated value
• make a positional command after -- clear the configured command path/args so the CLI override documented in setupArgv() actually wins
• add regression coverage for malformed UID/GID parsing, valid hex IDs, and config command override behavior

Security impact

This hardens privilege-boundary parsing. Before the change, malformed numeric ID strings could pass isANumber() and then be truncated by strtoimax(..., base 0), so values such as x or 0x became UID/GID 0. Also, a config exec_bin.path continued to win even when the caller provided a replacement command after --, contrary to the override behavior described in the code.

Verification

• make -j2
• git diff --cached --check
• focused checks: malformed -u x:x -g x:x now exits 255; 0x10 still runs as uid/gid 16; tests/command-override.cfg -- /bin/false exits 1
• make test passed through the new regression tests and then stopped at the existing pasta network test because pasta is not installed in this local environment