ip/tunnel: Abstract tunnel encapsulation options printing

ip/tunnel: Abstract tunnel encapsulation options printing

Get rid of code duplications and consolidate encapsulation
options printing in single function - tnl_print_encap().

Introduce and use tnl_encap_str() to format encapsulation
option string according to tempate and given values to avoid
code duplication and simplify it.

Use print_string() instead of fputs() and fprintf() to
print encapsulation for !is_json_context().

Print "unknown" parameter for "encap" type in PRINT_FP
context using "%s " format specifier and benefit from
complite time string merge.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>