conf, utils: moved my_popen()/my_pclose() to utils.c

Signed-off-by: Andrey Mazo <mazo@telum.ru>
utils: renamed and documented my_open()/my_close().

After my_open()/my_close() move from conf.c these functions should have
more meaningful names as now they may be used throughout the whole
codebase.
The involved struct deserves the same.

Signed-off-by: Andrey Mazo <mazo@telum.ru>
utils: add comment in popen_re_clearsigmask().

Signed-off-by: Andrey Mazo <mazo@telum.ru>
bdev: convert popen()/pclose() to popen_re_clearsigmask()/pclose_re_clearsigmask().

Signed-off-by: Andrey Mazo <mazo@telum.ru>