net: use internal/poll for DragonFly setKeepAlivePeriod
net: use internal/poll for DragonFly setKeepAlivePeriod

Fixes DragonFly build.

Change-Id: Id6b439cd4023ea8e3ed7cd9b70eec553c9eee4be
Reviewed-on: https://go-review.googlesource.com/36916
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,codestation/go,cloudflare/go,derekmarcotte/go,akutz/go,golang/go,cloudflare/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,linux-on-ibm-z/go,linux-on-ibm-z/go,josharian/go,derekmarcotte/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,linux-on-ibm-z/go,ImJasonH/go,Cofyc/go,golang/go,vsekhar/elastic-go,nawawi/go,ImJasonH/go,josharian/go,vsekhar/elastic-go,josharian/go,polyverse-security/go,nawawi/go,codestation/go,codestation/go,derekmarcotte/go,polyverse-security/go,linux-on-ibm-z/go,cloudflare/go,CAFxX/go,linux-on-ibm-z/go,codestation/go,cloudflare/go,CAFxX/go,nawawi/go,Cofyc/go,nawawi/go,CAFxX/go,golang/go,akutz/go,nawawi/go,derekmarcotte/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,golang/go,josharian/go,Cofyc/go,polyverse-security/go,akutz/go,nawawi/go,Cofyc/go,vsekhar/elastic-go,akutz/go,codestation/go,codestation/go,cloudflare/go,codestation/go,vsekhar/elastic-go,akutz/go,josharian/go,cloudflare/go,akutz/go,derekmarcotte/go,codestation/go,vsekhar/elastic-go,polyverse-security/go,josharian/go,Cofyc/go,ImJasonH/go,polyverse-security/go,linux-on-ibm-z/go,linux-on-ibm-z/go,polyverse-security/go,ImJasonH/go,josharian/go,derekmarcotte/go,akutz/go,derekmarcotte/go,ImJasonH/go,Cofyc/go,Cofyc/go,golang/go,ImJasonH/go,Cofyc/go,linux-on-ibm-z/go,CAFxX/go,derekmarcotte/go,vsekhar/elastic-go,christopher-henderson/Go,ImJasonH/go,golang/go,cloudflare/go,CAFxX/go,polyverse-security/go,ImJasonH/go