Fix mount/umount commands: use proper syscall wrappers and error messages

Fix mount/umount commands: use proper syscall wrappers and error messages

mount.c: Replace direct _syscall3 call with mount() wrapper from
sys/mount.h. Print strerror(errno) instead of raw -1 return value
from __syscall_ret, which was always -1 and unhelpful for debugging.

umount.c: Replace stub that always printed 'operation not supported'
with a working implementation using the umount() wrapper from
sys/mount.h. The SYS_UMOUNT2 syscall existed in the kernel but the
umount command never used it.

Tests: 103/103 PASS