Move parts of inferior job control to common/

Move parts of inferior job control to common/

This commit moves a few bits responsible for dealing with inferior job
control from GDB to common/, which makes them available to gdbserver.
This is necessary for the upcoming patches that will share
fork_inferior et al between GDB and gdbserver.

We move some parts of gdb/terminal.h to gdb/common/common-terminal.h,
especifically the code that checks terminal features and that are used
to set job_control accordingly.

After sharing parts of gdb/terminal.h, we also to share the two
functions on gdb/inflow.c that are going to be needed by the
fork_inferior rework.  They are 'gdb_setpgid' and the new
'have_job_control'.  I've also taken the opportunity to give a more
meaningful name to "inflow.c" on common/.  Now it is called
"job-control.c" (thanks Pedro for the suggestion).

gdb/ChangeLog:
2017-06-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILES): Add "common/job-control.c".
	(HFILES_NO_SRCDIR): Add "common/job-control.h".
	(COMMON_OBS): Add "job-control.o".
	* common/job-control.c: New file, with contents from
	"gdb/inflow.c".
	* common/job-control.h: New file, with contents from "terminal.h".
	* fork-child.c: Include "job-control.h".
	* inflow.c: Include "job-control.h".
	(gdb_setpgid): Move to "common/common-inflow.c".
	(_initialize_inflow): Move setting of "job_control" to
	"handle_job_control".
	* terminal.h (job_control): Moved to "common/common-terminal.h".
	(gdb_setpgid): Likewise.
	* top.c: Include "job_control.h".
	* utils.c: Likewise.
	(job_control): Moved to "job-control.c".

gdb/gdbserver/ChangeLog:
2017-06-07  Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (SFILE): Add "common/job-control.c".
	(OBS): Add "job-control.o".