Add experimental RS-485 support

Many modern RS-485 serial devices such as the ones from FTDI already
operate in RS-485 mode by default and will work with tio out of the box.
However, there are some RS-232/485 devices which need to be switched
from e.g. RS-232 to RS-485 mode to operate accordingly on the physical
level.

This commit implements the switching mechanism and interface required to
enable RS-485 mode. It only works on Linux and with serial devices which
use device drivers that support the Linux RS-485 control interface.

The RS-485 feature is detailed via the following options:

 --rs-485                           Enable RS-485 mode
 --rs-485-config <config>           Set RS-485 configuration

Set the RS-485 configuration using the following key or key value pair
format in the configuration field:

 RTS_ON_SEND=value             Set logical level (0 or 1) for RTS pin when sending
 RTS_AFTER_SEND=value          Set logical level (0 or 1) for RTS pin after sending
 RTS_DELAY_BEFORE_SEND=value   Set RTS delay (ms) before sending
 RTS_DELAY_AFTER_SEND=value    Set RTS delay (ms) after sending
 RX_DURING_TX                  Receive data even while sending data

If defining more than one key or key value pair, they must be comma
separated.

Example use:

 $ tio /dev/ttyUSB0 --rs-485 --rs-r485-config=RTS_DELAY_AFTER_SEND=50,RX_DURING_TX

src/meson.build

@@ -14,7 +14,8 @@ tio_sources = [
   'configfile.c',
   'signals.c',
   'socket.c',
-  'setspeed.c'
+  'setspeed.c',
+  'rs485.c'
 ]
 
 tio_dep = dependency('inih', required: true,
@@ -31,6 +32,10 @@ if enable_iossiospeed
   tio_c_args += '-DHAVE_IOSSIOSPEED'
 endif
 
+if enable_rs485
+  tio_c_args += '-DHAVE_RS485'
+endif
+
 executable('tio',
   tio_sources,
   c_args: tio_c_args,