Improve --help option for waf

As noted in #10 (closed) it is not clear that some waf options are for configure rather than build.

Also there are a couple of errors in the help. I propose the following changes to wscript (version attached).

46,47c46,47
< 	grp.add_option('--disable-dns-retry', action='store_true', default=False, help="Disable retrying DNS lookups.")
< 	grp.add_option('--disable-mdns-registration', action='store_true', default=False, help="Disable MDNS registration.")
---
> 	grp.add_option('--disable-dns-retry', action='store_true', default=False, help="Disable DNS lookups.")
> 	grp.add_option('--disable-mdns-registration', action='store_true', default=False, help="Disable DNS lookups.")
50c50
< 	grp = ctx.add_option_group("NTP features (specified during './waf configure')")
---
> 	grp = ctx.add_option_group("NTP features")
55c55
< 	grp = ctx.add_option_group("Refclock options (specified during './waf configure')")
---
> 	grp = ctx.add_option_group("Refclock options")
59c59
< 	grp = ctx.add_option_group("NTP developer options (specified during './waf configure')")
---
> 	grp = ctx.add_option_group("NTP developer options")
68c68
< 	grp = ctx.add_option_group("NTP documentation options (specified during './waf configure')")
---
> 	grp = ctx.add_option_group("NTP documentation options")