[Ohrrpgce] SVN: teeemcee/8461 Add Unix checked_system() to check return value of system(), use in safe

[subversion at HamsterRepublic.com]

teeemcee
2017-02-28 07:12:51 -0800 (Tue, 28 Feb 2017)
151
Add Unix checked_system() to check return value of system(), use in safe_shell()

Also avoid a warning in open_process() due to ignored system() return
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U   wip/os.bi
U   wip/os_unix.c
U   wip/util.bas