Add a libexec/ directory and use it to find the kak command

Add that libexec directory to the PATH instead of the current kak
binary directory to avoid impacting other commands.

The libexec directory currently only contains a symlink back to
the Kakoune binary.

libexec/kak

@@ -0,0 +1 @@
+../src/kak

src/Makefile

@@ -55,6 +55,7 @@ PREFIX ?= /usr/local
 DESTDIR ?= # root dir
 
 bindir := $(DESTDIR)$(PREFIX)/bin
+libexecdir := $(DESTDIR)$(PREFIX)/libexec/kak
 sharedir := $(DESTDIR)$(PREFIX)/share/kak
 docdir := $(DESTDIR)$(PREFIX)/share/doc/kak
 mandir := $(DESTDIR)$(PREFIX)/share/man/man1
@@ -162,6 +163,7 @@ distclean: clean
 
 installdirs:
 	install -d $(bindir) \
+		$(libexecdir) \
 		$(sharedir)/rc \
 		$(sharedir)/colors \
 		$(sharedir)/doc \
@@ -170,6 +172,7 @@ installdirs:
 
 install: kak man installdirs
 	install -m 0755 kak $(bindir)
+	ln -rsf $(bindir)/kak $(libexecdir)/kak
 	install -m 0644 ../share/kak/kakrc $(sharedir)
 	install -m 0644 ../doc/pages/*.asciidoc $(sharedir)/doc
 	cp -r ../rc/* $(sharedir)/rc

src/shell_manager.cc

@@ -72,7 +72,7 @@ ShellManager::ShellManager(ConstArrayView<EnvVarDesc> builtin_env_vars)
     // have access to the kak command regardless of if the user installed it
     {
         const char* path = getenv("PATH");
-        auto new_path = format("{}:{}", path, split_path(get_kak_binary_path()).first);
+        auto new_path = format("{}../libexec/kak:{}", split_path(get_kak_binary_path()).first, path);
         setenv("PATH", new_path.c_str(), 1);
     }
 }