configure.ac: Add build system test for readline

Test for readline presence and configuration time options controlling
readline use in rigctl and rotctl.
rigctl_parse.c: Implement readline interactive mode

Initial implementation of Readline input handling.  Only if 'configure'
finds Readline will it be enabled and when enabled it will only be used
by rigctl in interactive mode.  Passing rig commands from the rigctl
command line and rigctld use the original input handling which has not
been modified.
rigctl_parse.c: Implement history recall

Implement first cut at storing and recalling history.  At this point
history is retained for the current session only.

History is stored as complete command lines even if values are entered
at separate prompts.  Readline allows editing and even deleting recalled
history lines.
rigctl.c:  Add options to save/read command history

As commands from a previous session may not be desired, make reading the
history file at rigctl start and writing to it at rigctl close optional
by use of the -i/--read_history or -I/--save-history options.
Compilation is conditional on having Readline and Readline History
support detected at build system configuration time.

History is stored in '$HOME/.rigctl_history' by default.  An alternate
path may be specified by setting the RIGCTL_HIST_DIR environment
variable:

$ RIGCTL_HIST_DIR=~/.rigctl rigctl -iI

will temporarily set the history directory to /home/USER/.rigctl and
create .rigctl_history if it does not exist.  If the file cannot be read
or written a warning message will be given on stderr.
rigctl.1: Document Readline and history additions

Document Readline and history additions to rigctl along with new
-i/--read-history and -I/--save-history options.

Document use of RIGCTL_HIST_DIR environment variable to set an alternate
path for the .rigctl_history file.

Other minor edits.
rotctl_parse.c: Implement readline interactive mode

Initial implementation of Readline input handling.  Only if 'configure'
finds Readline will it be enabled and when enabled it will only be used
by rotctl in interactive mode.  Passing rotator commands from the rotctl
command line and rotctld use the original input handling which has not
been modified.
rotctl_parse.c: Implement history recall

Implement first cut at storing and recalling history.  At this point
history is retained for the current session only.

History is stored as complete command lines even if values are entered
at separate prompts.  Readline allows editing and even deleting recalled
history lines.
rotctl.c:  Add options to save/read command history

As commands from a previous session may not be desired, make reading the
history file at rotctl start and writing to it at rotctl close optional
by use of the -i/--read_history or -I/--save-history options.
Compilation is conditional on having Readline and Readline History
support detected at build system configuration time.

History is stored in '$HOME/.rotctl_history' by default.  An alternate
path may be specified by setting the ROTCTL_HIST_DIR environment
variable:

$ ROTCTL_HIST_DIR=~/.rotctl rotctl -iI

will temporarily set the history directory to /home/USER/.rotctl and
create .rotctl_history if it does not exist.  If the file cannot be read
or written a warning message will be given on stderr.
rotctl.1: Document Readline and history additions

Document Readline and history additions to rotctl along with new
-i/--read-history and -I/--save-history options.

Document use of ROTCTL_HIST_DIR environment variable to set an alternate
path for the .rotctl_history file.

Other minor edits.
Add Readline option to README files
Merge branch 'readline'

Merging branch 'readline' into master due to no complaints or other feedback.
Adds Readline capability for the command prompts in rigctl and rotctl.