OpenBSD: honor doas user for mount ownership and FUSE access

VeraCrypt derives the real (non-root) user from SUDO_UID/SUDO_GID
to set default mount-point ownership and the FUSE service access
filter. On OpenBSD, privileged commands are normally run through doas,
which exposes the invoking login name via DOAS_USER and does not set
the sudo variables. As a result, VeraCrypt launched through doas
attributes both to root instead of the invoking user.

When the sudo identity variables are absent, resolve DOAS_USER through
the password database and use that uid/gid for default mount-point
ownership and the VeraCrypt FUSE service access filter. sudo behavior
is unchanged.

This is a correctness fix for the doas launch path. It is not confirmed
to resolve the non-root ext2fs EACCES reported in the linked issues:
that failure occurs at the ext2fs layer reached through vnd, whose
backing-image I/O runs as root and is therefore already permitted by
the access filter.

Refs #1589.
Refs #1593.

src/Core/Unix/CoreUnix.cpp

@@ -21,6 +21,9 @@
 #ifdef TC_LINUX
 #include <sys/utsname.h>
 #endif
+#ifdef TC_OPENBSD
+#include <pwd.h>
+#endif
 #include <stdio.h>
 #include <unistd.h>
 #include "Platform/FileStream.h"
@@ -40,6 +43,26 @@ namespace VeraCrypt
 	static bool SamePath (const string& path1, const string& path2);
 #endif
 
+#ifdef TC_OPENBSD
+	static bool GetDoasUserIds (uid_t *uid, gid_t *gid)
+	{
+		const char *env = getenv ("DOAS_USER");
+		if (!env || !env[0])
+			return false;
+
+		struct passwd *pw = getpwnam (env);
+		if (!pw)
+			return false;
+
+		if (uid)
+			*uid = pw->pw_uid;
+		if (gid)
+			*gid = pw->pw_gid;
+
+		return true;
+	}
+#endif
+
 	// Struct to hold terminal emulator information
 	struct TerminalInfo {
 		const char* name;
@@ -634,6 +657,12 @@ namespace VeraCrypt
 			catch (...) { }
 		}
 
+#ifdef TC_OPENBSD
+		gid_t doasGid;
+		if (GetDoasUserIds (nullptr, &doasGid))
+			return doasGid;
+#endif
+
 		return getgid();
 	}
 
@@ -650,6 +679,12 @@ namespace VeraCrypt
 			catch (...) { }
 		}
 
+#ifdef TC_OPENBSD
+		uid_t doasUid;
+		if (GetDoasUserIds (&doasUid, nullptr))
+			return doasUid;
+#endif
+
 		return getuid();
 	}