Add followsymlinks flag to is-file, is-dir, is-blockdev, is-chardev, is-fifo and is-socket APIs.

This adds an extra optional boolean 'followsymlinks' flag to those 6
is-* APIs.  If the flag is true, then symlinks are followed, ie. we
use stat instead of lstat in the test.

For the rationale behind this change, see:
https://bugzilla.redhat.com/show_bug.cgi?id=974489
inspection: Fix inspection of Fedora guests (RHBZ#974489).

Commit e71b2c11f59b3f8ae0c4b31c4ab5b0d1bfcdf181 broke inspection of
Fedora guests because guestfs_is_file returns false for
/etc/redhat-release (it's a symlink to a file, not a file).

We fix this by using the new followsymlinks flag added in the
previous commit.  Thus guestfs_is_file becomes
guestfs_is_file_opts (g, filename, GUESTFS_IS_FILE_OPTS_FOLLOWSYMLINKS, 1, -1)
which checks if it's a file or a symlink to a file.

This fixes commit e71b2c11f59b3f8ae0c4b31c4ab5b0d1bfcdf181.