confile: use mmap() to parse config file

confile: use mmap() to parse config file

Sigh, this is going to be fun. Essentially, dynamic memory allocation through
malloc() and friends is unsafe when fork()ing in threads. The locking state
that glibc maintains internally might get messed up when the process that
fork()ed calls malloc or calls functions that malloc() internally. Functions
that internally malloc() include fopen(). One solution here is to use open() +
mmap() instead of fopen() + getline().

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>