Use unlocked libc IO everywhere.

Since mutt does not use threads, there is no reason it should use the
locked variants of the FILE* IO functions. This checks if the unlocked
functions are available, and if so enables them globally via mutt.h.

Cuts load time for a 56k message, 1.8GB /var/mail mailbox from 14
seconds to ~6 seconds, since we avoid acquiring and releasing a mutex
for every character of input read.

Before: 0m14.376s

    74.98%          mutt  libc-2.18.so        [.] _IO_getc
    11.87%          mutt  mutt                [.] mbox_parse_mailbox
     0.94%          mutt  [kernel.kallsyms]   [k] copy_user_generic_string
     0.83%          mutt  libc-2.18.so        [.] __strchr_sse2
     0.53%          mutt  libc-2.18.so        [.] __memcpy_sse2
     0.44%          mutt  libc-2.18.so        [.] _int_malloc

After: 6 seconds

    68.92%     mutt  mutt                  [.] mbox_parse_mailbox
     2.25%     mutt  [kernel.kallsyms]     [k] copy_user_generic_string
     1.73%     mutt  libc-2.18.so          [.] __strchr_sse2
     1.24%     mutt  libc-2.18.so          [.] __memcpy_sse2
     1.17%     mutt  libc-2.18.so          [.] _int_malloc
     0.87%     mutt  libc-2.18.so          [.] __strspn_sse42