Check for mntent.h header in configure always

Don't skip the check for OS X.
Add mntent.h compat file for FreeBSD-like systems
Fix stupid mistake in configure related to mntent

Didn't replace all -z flags of test with -n.
Always check for strverscmp() in configure script
Remove MNT_NODEV flag from mntent.c

Seems like it's deprecated or something like that.
Remove several MNT_ flags from mntent.c for OS X
Add Daniel Dettlaff to the THANKS file

Helped with mntent() compatibility on FreeBSD and OS X and with
configuration on FreeBSD.
Fix segfault related to mntent compatibility file
Correct preprocessor directives for OS X
Correct one more preprocessor directive for OS X
Update documentation
Make code of is_on_slow_fs() a bit better

Use const qualifier for a variable.
Simplify resource cleanup and returning of result.
Don't parse "/etc/mtab" file if 'slowfs' is empty

I know this seems to be a micro optimization, but in fact this function
is called quite often.
Touch aclocal.m4 file

A workaround for autoconf to ease building after pulling commits, which
change configure.in script.
Merge branch 'fix-mntent-for-freebsd'
 Add sort option list assert to sort_dir_list
Fix documentation about column view

Add a missing space.
Specify that fields are aligned to the right by default (wrongly
mentioned left alignment as the default one).
Add an assert to redraw_sort_dialog about options

So it's harder to forget to update the dialog.
Remove an odd empty line at sort_dir_list()
A bit better code for get_perm_string() function

Use [] instead of *.
Use const and static modifiers for internal array.
Add function comment.