replace deprecated `index` with `strchr`

The `index` libc function was removed in POSIX 2008, and `strchr` is a direct
replacement. The bionic (Android) libc has removed `index` when you are
compiling for a 64-bit architecture, such as AArch64.

Signed-off-by: Patrick O'Leary <patrick.oleary@gmail.com>
lxcmntent: remove most bionic compatability

Newer Android platforms have a version of bionic libc which supports
most of these functions.

Signed-off-by: Patrick O'Leary <patrick.oleary@gmail.com>
build: make position-independent executables

Android 5.0+ (Lollipop) will not execute native applications that are not
built as position-independent executables.

Signed-off-by: Patrick O'Leary <patrick.oleary@gmail.com>