Fix various posix shell compliance issues in the rc/ files

rc/core/man.kak

@@ -37,7 +37,7 @@ def -hidden -params .. _man %{ %sh{
 
 def -params .. \
   -shell-completion %{
-    prefix=${1:0:${kak_pos_in_token}}
+    prefix=$(echo "$1" | cut -c1-${kak_pos_in_token} 2>/dev/null)
     for page in /usr/share/man/*/${prefix}*.[1-8]*; do
         candidate=$(basename ${page%%.[1-8]*})
         pagenum=$(printf %s "$page" | sed -r 's,^.+/.+\.([1-8][^.]*)\..+$,\1,')
@@ -52,7 +52,7 @@ def -params .. \
     pagenum=""
 
     ## The completion suggestions display the page number, strip them if present
-    if [[ $subject =~ [a-zA-Z_-]+\([^\)]+\) ]]; then
+    if expr "$subject" : '[a-zA-Z_-]+\([^\)]+\)'; then
         pagenum=${subject##*\(}
         pagenum=${pagenum:0:$((${#pagenum} - 1))}
         subject=${subject%%\(*}

rc/extra/clang.kak

@@ -37,7 +37,7 @@ def clang-parse -params 0..1 -docstring "Parse the contents of the current buffe
                 *) ft=c++ ;;
             esac
 
-            if [ "$1" == "-complete" ]; then
+            if [ "$1" = "-complete" ]; then
                 pos=-:${kak_cursor_line}:${kak_cursor_column}
                 header="${kak_cursor_line}.${kak_cursor_column}@${kak_timestamp}"
                 compl=$(clang++ -x ${ft} -fsyntax-only ${kak_opt_clang_options} \

rc/extra/git-tools.kak

@@ -29,7 +29,7 @@ def -params 1.. \
   -docstring %sh{printf "%%{Git wrapping helper\navailable commands:\n add\n rm\n blame\n commit\n checkout\n diff\n hide-blame\n log\n show\n show-diff\n status\n update-diff}"} \
   -shell-completion %{
     shift $(expr ${kak_token_to_complete})
-    prefix=${1:0:${kak_pos_in_token}}
+    prefix=$(echo "${1}" | cut -c1-${kak_pos_in_token} 2>/dev/null)
     (
       for cmd in add rm blame commit checkout diff hide-blame log show show-diff status update-diff; do
           expr "${cmd}" : "^\(${prefix}.*\)$"