connmgr: add nowatchdog go build tag

[birros]

Add the ability to disable watchdog even if cgo is enabled.

I logicaly tested the configuration using a truth table to cover each case:

p	q	p∧¬q	¬p∨q	(p∧¬q)⊕(¬p∨q)
T	T	F	T	T
T	F	T	F	T
F	T	F	T	T
F	F	F	T	T

Where:

• p: cgo tag
• q: nowatchdog tag
• p∧¬q: watchdog_cgo.go compiled if true
• ¬p∨q: watchdog_no_cgo.go compiled if true
• (p∧¬q)⊕(¬p∨q): only one of these files is compiled if true

Usage (for newbies)

$ go build -tags nowatchdog .

Additionnal note

This fixes an old problem I reported a few months ago: libp2p/go-libp2p-connmgr#98.

I couldn't find a convention to write a deactivation tag, so I added the no prefix to watchdog but if you have a better proposal you're welcome.