Support relative #include, #warning and #error directives #59

Job	Run time
tests-exe (i386_linux, ubuntu-latest)	3s
tests-exe (x86_64_linux, ubuntu-latest)	3s
bootstrap-pnut-exe (bash, i386_linux)	15s
tests-exe (x86_64_mac, macos-latest)	8s
bootstrap-pnut-exe (bash, x86_64_linux)	8s
bootstrap-pnut-sh (bash)	10s
bootstrap-pnut-exe (bash, x86_64_mac)	10s
bootstrap-pnut-exe (dash, i386_linux)	11s
bootstrap-pnut-sh (dash)	12s
tests-shell (bash)	15s
bootstrap-pnut-exe (dash, x86_64_linux)	13s
bootstrap-pnut-sh (ksh)	13s
tests-shell (dash)	10s
bootstrap-pnut-sh (mksh)	28s
bootstrap-pnut-exe (dash, x86_64_mac)	9s
tests-shell (ksh)	13s
bootstrap-pnut-sh (yash)	12s
bootstrap-pnut-exe (ksh, i386_linux)	13s
tests-shell (mksh)	25s
bootstrap-pnut-sh (zsh)	14s
bootstrap-pnut-exe (ksh, x86_64_linux)	12s
tests-shell (yash)	14s
bootstrap-pnut-exe (ksh, x86_64_mac)	9s
tests-shell (zsh)	22s
success-message	0s
	5m 2s

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