function: Add explicit show command

function: Add explicit show command

Using default behavior of 'gt func' command as only way of showing
functions is solution far from ideal. E.g. in following example second
command is ambiguous:

	$ gt create rm
	$ gt func rm

Even if gadget names like 'rm' will not be commonly used, we cannot forbid
them. To prevent situations like that, this commit introduce  explicit
'func show' command for showing functions.

Previous default behavior for 'gt func' command is preserved.

Change-Id: Ifa5d5fbb3e78174cfb69231da7272ad8fa72c568
Signed-off-by: Pawel Szewczyk <p.szewczyk@samsung.com>