quitcd.bash_zsh: make POSIX compliant; minor fixes and style changes

This commit makes the following changes to quitcd.bash_zsh:
- POSIX compliance and an according rename of the script
- Enforces consistent if-then statements and indentation
- Minor comment fixes

Signed-off-by: J. Brock <joseph.brock@protonmail.com>

misc/quitcd/quitcd.bash_sh_zsh

@@ -1,10 +1,10 @@
 n ()
 {
     # Block nesting of nnn in subshells
-    if [[ "${NNNLVL:-0}" -ge 1 ]]; then
+    [ "${NNNLVL:-0}" -eq 0 ] || {
         echo "nnn is already running"
         return
-    fi
+    }
 
     # The behaviour is set to cd on quit (nnn checks if NNN_TMPFILE is set)
     # If NNN_TMPFILE is set to a custom path, it must be exported for nnn to
@@ -19,12 +19,12 @@ n ()
     # stty lwrap undef
     # stty lnext undef
 
-    # The backslash allows one to alias n to nnn if desired without making an
-    # infinitely recursive alias
-    \nnn "$@"
+    # The command builtin allows one to alias nnn to n, if desired, without
+    # making an infinitely recursive alias
+    command nnn "$@"
 
-    if [ -f "$NNN_TMPFILE" ]; then
+    [ ! -f "$NNN_TMPFILE" ] || {
         . "$NNN_TMPFILE"
         rm -f "$NNN_TMPFILE" > /dev/null
-    fi
+    }
 }