orbea/exists: Posix shell function for finding executable files in the $PATH

Posix shell function for finding executable files in the $PATH

orbea ec41715f06 Use test -n everywhere		2 lat temu
scripts	75426af2f1 test.sh: Test if command exits with the first missing arg.	4 lat temu
.gitignore	508406ee4e exists.sh: Return absolute paths.	4 lat temu
LICENSE	ec41715f06 Use test -n everywhere	2 lat temu
TODO	69f7c7a66a Add a TODO.	4 lat temu
command.sh	ec41715f06 Use test -n everywhere	2 lat temu
exists.sh	ec41715f06 Use test -n everywhere	2 lat temu
run.sh	ec41715f06 Use test -n everywhere	2 lat temu
test.sh	ec41715f06 Use test -n everywhere	2 lat temu
which.sh	ec41715f06 Use test -n everywhere	2 lat temu