MI: Add -a option to the "-data-disassemble" command

MI: Add -a option to the "-data-disassemble" command

The CLI "disassemble" command allows specifying a single address - in
that case the function surrounding that address is disassembled.

This commit adds this feature to the equivalent MI command
"-data-disassemble".

gdb/ChangeLog:
2018-08-14  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* mi/mi-cmd-disas.c (mi_cmd_disassemble): Add -a option.
	If used, use find_pc_partial_function to find address range
	to disassemble.
	* mi/mi-main.c (mi_cmd_list_features): Report
	"data-disassemble-a-option" feature.
	* NEWS: Mention new -data-disassemble option -a.

gdb/doc/ChangeLog:
2018-08-14  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* gdb.texinfo (GDB/MI Data Manipulation): Document
	"-data-disassemble -a addr".
	(GDB/MI Support Commands): Document "data-disassemble-a-option"
	feature.

gdb/testsuite/ChangeLog:
2018-08-14  Jan Vrany  <jan.vrany@fit.cvut.cz>

	* gdb.mi/mi-disassemble.exp (test_disassembly_only): Add tests for
	-data-disassemble -a.
	(test_disassembly_bogus_args): Likewise.