install-font.sh -P needs to allow dots in its argument.

[T. Kurt Bond]

The linux version of homebrew installs groff's data directory in
/home/linuxbrew/.linuxbrew/share/groff by default.

I must have cargo culted the check_optarg without checking everything out
when I added the -P option.  I've attached a patch to install-font.sh to
make it check that the argument to -P is a directory and that it is
writable.

I kept the check that the argument doesn't start with a dash, even though
someone evil might start their directory with a dash.  Should that check be
dropped?

-- 
T. Kurt Bond, tkurtb...@gmail.com, https://tkurtbond.github.io

--- install-font.sh.orig	2020-08-02 07:40:54.598224649 -0400
+++ install-font.sh	2020-08-02 08:30:09.915547433 -0400
@@ -223,6 +223,18 @@ Aborting.\n" >&2
 Aborting.\n" >&2
       exit 1
     ;;
+    not_dir)
+      printf \
+"[install-font]: \"${OPTARG}\" not a directory.  \
+Aborting.\n" >&2
+      exit 1
+    ;;
+    not_writable)
+      printf \
+"[install-font]: \"${OPTARG}\" is not writable.  \
+Aborting.\n" >&2
+      exit 1
+    ;;
     incompatible)
       printf \
 "[install-font]: Incompatible file type. Must have extension ttf|pfa|pfb|otf.  \
@@ -465,6 +477,16 @@ check_optarg()
   :
 }
 
+check_optarg_dir()
+{
+  [ "${OPTARG:0:1}" = "-" ] && error no_arg # is an opt when arg to opt expected
+
+  [ ! -d "${OPTARG}" ] && error not_dir
+
+  [ ! -w "${OPTARG}" ] && error not_writable
+  :
+}
+
 while getopts ":C:cndDF:f:hHlpP:s" opt
 do
   case "$opt" in
@@ -519,7 +541,7 @@ do
     ;;
     P)
       option=P
-      check_optarg
+      check_optarg_dir
       loc_or_sysdir=${OPTARG}
     ;;
     p)