imfc c-grate bugs

[rennergade]

imfs_grate.c:

• register_handler wrong arg count
• unlink_grate leaks pathname; pwrite_grate/write_grate leak buffer

imfs.c:

• imfs_fcntl switches on fd instead of op
• imfs_opendir NULL dereference (no malloc)
• __imfs_writev passes count where offset should go
• remove_child uninitialized remove_idx
• imfs_readdir off-by-one null terminator
• SEEK_HOLE/SEEK_DATA broken pointer arithmetic on Node*
• load_file opens FILE handle, never closes it
• read_full_file no NULL checks on fopen/malloc

imfs.h:

• missing include guard; static int PC_CONSTS[] defined in header