[MNG-7270] Switch to shell alternative to "which"

In some circumstances the init script calls `which`, which may or may not be
available on the host system. Instead, use `command -v`, which is nearly
equivalent. One area it differs is if the command being queried is defined as
a shell alias. To avoid that, call `unset -f command` to avoid the situation
where "command" has been re-defined as a shell function.

See here for more information on this approach:
<https://pubs.opengroup.org/onlinepubs/009695399/utilities/command.html>

Tested with bash, sh (bash invoked as sh), posh, dash, zsh and mksh.

This closes #556

apache-maven/src/assembly/shared/init

@@ -20,7 +20,7 @@ if [ -n "$JAVA_HOME" ] ; then
     fi
   fi
 else
-  JAVACMD="`which java`"
+  JAVACMD="`\\unset -f command; \\command -v java`"
 
   if [ ! -x "$JAVACMD" ] ; then
     echo "The java(1) command does not exist in PATH nor is JAVA_HOME set, so Apache Maven cannot be started." >&2