Ticket #2944: code cleanup before 4.8.8 release.

(mc_prompt): move from src/setup.[ch] to src/filemanager/midnight.[ch].

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(read_subshell_prompt): return gboolean instead of int.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(subshell_prompt): changed to GString.

(read_subshell_prompt): refactoring to ret rid of low-level memory reallocation.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
(mc_global_t::shell): new member to store user's shell

...instead of global variable "shell".

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Avoid C++-style comments.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Remove irrelevant comments about file_date() function.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge src/filemanager/option.[ch] to src/filemanager/boxes.[ch].

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Clarify usage of use_internal_edit variable

...and disable "Use internal edit" checkbox in case of build
without internal editor.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Enable -Wunused-but-set-variable flag by default.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Clarify check of C compiler flags.

AC_TRY_COMPILE macro is obsolete. Use AC_COMPILE_IFELSE instead.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>