Improve mount waiting + misc doc improvements

Adds user.mergerfs.branch and user.mergerfs.branch_mounts_here
Adds .mergerfs.branch and .mergerfs.branch_mounts_here files.

src/fs_mounts.cpp

@@ -0,0 +1,37 @@
+#include "fs_mounts.hpp"
+
+#include <cstdio>
+
+#include <mntent.h>
+
+#ifdef __linux__
+void
+fs::mounts(fs::MountVec &mounts_)
+{
+  FILE *f;
+
+  f = setmntent("/proc/mounts","r");
+  if(f == NULL)
+    return;
+
+  struct mntent *entry;
+  while((entry = getmntent(f)) != NULL)
+    {
+      fs::Mount m;
+
+      m.dir    = entry->mnt_dir;
+      m.fsname = entry->mnt_fsname;
+      m.type   = entry->mnt_type;
+      m.opts   = entry->mnt_opts;
+
+      mounts_.emplace_back(std::move(m));
+    }
+
+  endmntent(f);
+}
+#else
+void
+fs::mounts(fs::MountVec &mounts_)
+{
+}
+#endif