Fix get_full_path(), handle newfstatat

[krs85]

get_full_path():

• (at) versions of syscalls: AT_FDCWD or resolve the dir_fd they provide.
• ./
• ../
• /already/full/path
• just_file_name.txt
• implement for rename/renameat?
• pass file arg in in addition to the RcExecution? Would help me handle rename.

Also handle newfstatat. Basically the same as handling stat, just fancier get_full_path() nonsense for it.

• Intercept newfstatat.
• Basically treat it the same way as stat but obviously the paths aren't treated the same way.