Fix compilation on musl by implementing a custom basename

[Stargirl-chan]

This PR fixes #389. Tested on a glibc and musl system.

Changes

• Implemented a custom basename() version: basenm() using the simple implementation of it that was present in maybe_fopen_alias()
• Modified the simple implementation of it to return a copy of the argument that was given, instead of NULL if no / was present
• Changed all calls to basename() to our custom version
• Changed the comparison in maybe_fopen_alias() as we no longer return NULL