Handle unset MYVIFMRC envvar in is_old_config()

Also add function description in the appropriate header file.
Handle unset MYVIFMRC envvar in exec_config()

Also add function description in the appropriate header file.
Rename exec_config() to source_config()

New name reflects better what that function is really doing.
Introduce DEFAULT_SORT_KEY

This serves two purposes:

  * to eliminate ifdef directives in several places;
  * to make code more readable.
Fix buffer overflow on composing list of sort keys

Used to happen only for very long lists.

Thanks to Christoph (a.k.a. informationen).
Add read_line() function to read very long lines

This new function doesn't have size limits.  It just reads next part of
line and adds it to the buffer after reallocating it.  The function
should help to solve issues with buffers of predefined size.
Use read_line() function in read_info_file()

This solves issues on reading very long lines from vifminfo file on
startup.
Use read_line() function in update_info_file()

This solves issues on reading very long lines from vifminfo file on
writing (updating) it.
Use GNU/POSIX getline() function if available

Use it to implement read_line(), which then just removes trailing
newline character and does all hard work of getline() on systems that
don't have it.
Touch aclocal.m4 file

A workaround for autoconf to ease building after pulling commits, which
change configure.in script.
Remove MAX_LEN macro from info.c file

For this replace automatic character buffer with dynamic buffer.

Also update stdio.h include comment in file_streams.c file.
Change prepare_line to remove_leading_whitespace

Now read_line() removes trailing newline character, so we can simplify
prepare_line() function.
Add read_vifminfo_line() function to cfg/info.c

    This simplifies reading of lines from vifminfo file.
Fix memory leak in getline() used from read_line()
Use read_line() function in is_conf_file()
Small is_old_config() enhancements

Move it inside the file.
Add forward declaration.
Add function comment.
Use `var_name[]` instead of `*var_name`.
Partially use read_line() in source_file_internal

Remove one of buffers with predefined size.
Increase size of the other one.
Use better name for a constant.
Put notice about very long lines fix to ChangeLog

Thanks to Christoph (a.k.a. informationen).
Merge branch 'fix-conf-file-long-lines'