Changed hyphen in version string to underscore.

According to RFC 4253 the version string cannot include
a hyphen or minus sign in the protoversion or softwareversion
stanzas. So while you can use a hyphen between those stanzas
you cannot include a hypen in the stanza. Since the hpn version
is part of the softwareversion stanza we can't include a hypen.
Chnaging it to an underscore is acceptable and won't break
our compatability tests in compat.c
See RFC 4253 Section 4.2.

brought to our attention by Lapo Luchini via github
issue #97

version.h

@@ -3,5 +3,5 @@
 #define SSH_VERSION	"OpenSSH_9.8"
 
 #define SSH_PORTABLE	"p1"
-#define SSH_HPN         "-hpn18.5.0"
+#define SSH_HPN         "_hpn18.5.0"
 #define SSH_RELEASE	SSH_VERSION SSH_PORTABLE SSH_HPN