libbladeRF: Added gain hotkey support to repeater test program

The numeric keys may be used to increase/decrease gains while running.
libbladeRF: Minor touch-ups in test_repeater

No need to use dummy variable with bladerf_init_stream as of commit
3e89a44f66239dfeb01ca6c5642e8c87c38cbbe7.

Reduced default sample rate to 1 Msps to avoid timeouts on Hi-speed
connections.
libbladeRF: Bumped patch rev (v0.13.2)

Changes include:
 - Repeater test hotkey support and minor fixes
bladeRF-cli: Bumped patch rev (v0.10.6)

Changes include:
    - Removal of bladerf_close() call after bladerf_device_reset() call
bladeRF-cli: Fixed unclear help text
libbladeRF: Documentation touch-up in bladerf_stream description

Noted that bladerf_enable_module() *must* be called prior to calling
bladerf_stream().
host: Added check for libc version

It's necessary to build with -lrt for libc < 2.17 when using
clock_gettime().
libbladeRF: Re-implemented sync interface, updated API docs, renamed timeout function

The sync interface has been re-implemented atop the existing
asynchronous interface. This is currently opt-in and only enable when
pthreads support is enabled and available.

Some clarifications have been made in the API documentation,
specifically in the explanations of the various stream parameters.

The bladerf_set_transfer_timeout and bladerf_get_transfer_timeout have
been named more appropriately, as these control stream transfer
timeouts, as opposed to all transfers.
libbladeRF: libbladerf_test_sync runs full duplex
libbladeRF: Added loopback and clean SIGTERM handling to test_sync
libbladeRF: Moved internal sync handles into an array

This is a bit more consistent with what's already done with timeouts,
and is the preferred method of adding per-module items in the future.
libbladeRF: Include required OSX common/ directory items in build

The following items were missing from the changes introduced in commit
28ed798ce9b4a81f1bad6a6803a3d4dab3f146d3:

   - Add common/include/osx directory to libbladeRF include path
   - Add common/osx/src/clock_gettime.c to libbladeRF build
libbladeRF: [WIP] Started stubbing out metadata functionality
[WIP] Backing out some changes to redesign a bit
libbladeRF: [WIP] Added bladerf_enable_metadata() stub
libbladeRF: [WIP] Added some metadata handling to RX stream callback

Untested, likely very buggy
libbladeRF: [WIP] Added meta status+timestamp to sync RX call

Untested, likely a mess.