Like the other "at" functions, renameat renames files with respect with
a directory descriptor. "renameat2" is Linux specific but really nice -
it adds a flag which supports atomically swapping two files or failing
if the target already exists. The latter is easily supported in relibc.
The former requires redoxfs support.
Besides the impl itself, I refactored "cap_path_at" into what it really
is: a mini openat2-like function.
%m is a format specifier that prints an error string for errno. The
specifier is technically only for syslog, but musl and glibc implement
it for printf itself. Parsing for a single specifier in a single
function is error prone, especially when syslog itself is variadic.
nicoan