maint: trivial system header file cleanups

maint: trivial system header file cleanups

* src/system.h: Note where it should be included, and
make ordering check portable to GLIBC > 2
* src/copy.c: Move <sys/ioctl.h> along with other system headers
as is done elsewhere.
* src/install.c: Move <sys/wait.h> along with other system headers
as is done elsewhere.
* src/ptx.c: Include <regex.h> rather than "regex.h" as
is done elsewhere.  Note <regex.h> is kept after "system.h"
as per commit dba300a0.