Allow underscores in guifont PostScript names #991

Job	Run time
build-and-test (macos-14, 15.4, false) / build-and-test	12m 21s
build-and-test (macos-15, 16.4, true, true, true) / build-and-test	33m 10s
build-and-test (macos-13, 15.2, true, vimtags, check-xcodeproj-compat) / build-and-test	13m 30s
build-and-test (macos-13-xcode14-self-hosted, true, _10.9, true, true) / build-and-test	0s
	59m 1s

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