[SV 54740] Ensure .SILENT settings do not leak into sub-makes

[SV 54740] Ensure .SILENT settings do not leak into sub-makes

Create a new variable run_silent to hold the current instance's global
silence setting, allowing silent_flag to represent only whether the -s
option was provided on the command line.

* src/makeint.h: Change silent_flag variable to run_silent.
* src/job.c: Ditto.
* src/remake.c: Ditto.
* src/file.c: Ditto.
* src/main.c: Add a new global variable run_silent.
(decode_switches): After switches are decoded, initialize run_silent.
* tests/scripts/targets/SILENT: Add a test for recursive behavior.